71 Commando Head Issues?

nopdog · May 8, 2021

Hope you can help and advise.
I’ve just completed the fitting of new carbs to my 71 Commando 750. So carbs are working fine.
Bike is not smoking both cylinders have good spark but left cylinder is not firing and is leaking a lot of fuel out of junction between header pipe and exhaust.
Sounds to me like inlet valve is shot! Would this be correct?
Any recommendations of a Norton head specialist would be appreciated. I’m in South Coast NSW Australia.
Engine sounds very quiet but if I’ve got the head off what else should be looked at.

Thanks.

APRRSV · May 8, 2021

nopdog said:
Hope you can help and advise.
I’ve just completed the fitting of new carbs to my 71 Commando 750. So carbs are working fine.
Bike is not smoking both cylinders have good spark but left cylinder is not firing and is leaking a lot of fuel out of junction between header pipe and exhaust.
Sounds to me like inlet valve is shot! Would this be correct?
Any recommendations of a Norton head specialist would be appreciated. I’m in South Coast NSW Australia.
Engine sounds very quiet but if I’ve got the head off what else should be looked at.

Thanks.

Nopdog,
Are you sure the carbs are working fine? I wonder whether you have serious carb issue pouring too much fuel into your left cylinder.

Ed

worntorn · May 8, 2021

Float stuck or set too high?

Glen

nopdog · May 8, 2021

I’ll do a compression test.
what would an acceptable range be.
Yes they are new carbs.
I took them of and stripped them all looked ok. I could switch the bowls around.
But it is a lot of fuel!
Simon

nopdog · May 8, 2021

I’ll do a compression test.
what would an acceptable range be.
Yes they are new carbs.
I took them off and stripped them all looked ok. I could switch the bowls around.
But it is a lot of fuel!

Tornado · May 8, 2021

Something around 120 psi cold is reasonable....maybe closer to 150 psi warm.

Richard Tool · May 8, 2021

check the needle in the left carb - I once had the same issue - turns out I had not guided the needle into it’s bore properly when re assembling the carb .

Mart UK · May 9, 2021

And check the floats. Premiers come with the stay up floats, I think. I found a burr on the spindle of mine, which caused it to stick, but only when the bowl was screwed back on.

Bernhard · May 9, 2021

nopdog said:
But it is a lot of fuel!
Simon

Sounds like stuck needle in carb float bowl, with engine dead check to see if excess fuel comes out of carb.....or you could have forgotten to fit pilot jet, needle jet. .and so on .

johnm · May 9, 2021

Was the bike running before you changed the carbs? An inlet valve just dosnt fail like that.

And even if it did fuel should not run through the engine and out the exhaust.

To me it sounds like 95% chance something is sticking in that carb.

NIX · May 9, 2021

Try the easy things first like checking valve lash, swapping plugs/leads/caps/coils. Is the fuel also coming out of the tickler? If not, check fuel level in both bowls. It would surprise me if they are correct right out of the box. It takes some work as the procedure is not the simplest, but you need to rule out possible causes. Check if the needle closes correctly, even with an adjusted float level.

Fast Eddie · May 9, 2021

johnm said:
Was the bike running before you changed the carbs? An inlet valve just dosnt fail like that.

And even if it did fuel should not run through the engine and out the exhaust.

To me it sounds like 95% chance something is sticking in that carb.

Indeed. Starting point for problem solving should always be the simple things first AND whatever has been changed / is new in the equation. It might be something else, but we should always rule these out first.

If the bike ran well before, and then you fitted new carbs, and now it doesn’t, you should be looking at the carbs.

This is especially so when there are (sadly) known issues with these carbs from new, like: swarf, burrs on the floats, incorrect float heights, etc.

Its an unfortunate trait of motorcyclist nature that we always assume the worst!

nopdog · May 9, 2021

Thanks for that. All good advise.
I’ll recheck the carbs.

nopdog · May 10, 2021

Oh well. I feel like such a dick!!!
Turns out the air fuel mixture screw was turned all the way in.
All part of the rich tapestry of life I guess.
Again thanks for the help.

NIX · May 11, 2021

Good you found it and another time I'm confirmed in checking the easy things first. (Even though I didn't mention thisone.)

baz · May 11, 2021

nopdog said:
Oh well. I feel like such a dick!!!
Turns out the air fuel mixture screw was turned all the way in.
All part of the rich tapestry of life I guess.
Again thanks for the help.

No you are not a dick , you found the problem mate

Richard Tool · May 11, 2021

Glad it was something easy to fix - turning out a screw is a little easier than drifting out a valve guide ...

nopdog · May 21, 2021

Sorry fellows I have to revive this thread. Still not right.
Turns out the inline fuel filter was partially blocked as well as the pet cock on left hand side.
So no fuel left to left pot.
I've taken the carbs back off all looks ok but did not check/clean air fuel mixture screw.
Main jet is 220 pilot is 106.

When I got the bike it had NGK BP5ES spark plugs and balance pipe between the float bowls.
New carbs and BP7ES plugs and no balance pipe = excess fuel problem in left cylinder.
I've put the plugs back to the BP5ES and reinstated the balance pipe between the carbs.
The excess fuel through left pot has disappeared but now have a bad miss in left pot.
Even at idle it's seems to be firing occasionally in back of muffler/silencer. And missing bad/ firing on one pot under load.
I have reasonable/not blue spark to both plugs. I have not checked timing as yet. I have not balanced the carbs as per Bushmans as yet.
Fuel/air mixture at 1 1/2 turns out.
I'm running out of hair to pull out.

EstuaryBoy · May 21, 2021

nopdog said:
Sorry fellows I have to revive this thread. Still not right.
Turns out the inline fuel filter was partially blocked as well as the pet cock on left hand side.
So no fuel left to left pot.
I've taken the carbs back off all looks ok but did not check/clean air fuel mixture screw.
Main jet is 220 pilot is 106.

When I got the bike it had NGK BP5ES spark plugs and balance pipe between the float bowls.
New carbs and BP7ES plugs and no balance pipe = excess fuel problem in left cylinder.
I've put the plugs back to the BP5ES and reinstated the balance pipe between the carbs.
The excess fuel through left pot has disappeared but now have a bad miss in left pot.
Even at idle it's seems to be firing occasionally in back of muffler/silencer. And missing bad/ firing on one pot under load.
I have reasonable/not blue spark to both plugs. I have not checked timing as yet. I have not balanced the carbs as per Bushmans as yet.
Fuel/air mixture at 1 1/2 turns out.
I'm running out of hair to pull out.

'.........no balance pipe = excess fuel problem in left cylinder.'

Points to unregulated or not properly regulated supply... I.e: float height, float free movement or needle seat.

Mart UK · May 21, 2021

Have you checked the float on the carb that side for level/sticking, as mentioned in an earlier post?

71 Commando Head Issues?

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